Causal FIR Filters

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From Eq. (B.2),
we that the
impulse response is alwayszero for .
Any filter
having a zero impulse response prior to time is said
to be causal. Thus, a tapped delay line such as that depicted in
Fig. B.1
can only implement causal FIR
filters. In software, however, we may easily implement
non-causal
filters as well based simply on the definition ofconvolution.
However, noncausal filters are never precisely physical, of course,
since events are causal in the real world (disregarding certain
effects in quantum mechanics).